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Old 14th August 2009, 02:09 PM   #1
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Default DC 0ffset input pair

Hi to All


I am with a question, there any way to reduce the DC offset in the differential pair? This issue is addressed in book of Douglas Self (Audio Power Amplifier Design Handbook, Third edition, PAG 85-89)

In my design amp had to use high-beta TRs in input pair, , even so, have limited input impedance in 20K and the DC offset is 32mA (Fig attached), according to what is in the book of Self, the current generates noise(blameless amp of Self, has only 10K input impedance).

PS:I do not have DC offset in output (<1mA).

Thanks
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File Type: gif dc_offset.gif (3.5 KB, 231 views)
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Old 14th August 2009, 02:25 PM   #2
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Is that the simulatedvalue of base current?

If so it is impossible!

Is R26/R32 junction ground?

Need to post some voltages and show full schematic
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Old 14th August 2009, 02:54 PM   #3
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Quote:
Originally posted by cliff
Is that the simulatedvalue of base current?
If so it is impossible!
Yes, must simulate funcion .OP then just put the cursor on top node, the value appears in the bottom bar of the simulator.
Quote:
Originally posted by cliff
Is R26/R32 junction ground?
[/B]
Yes (attached)
Quote:
Originally posted by cliff
Need to post some voltages and show full schematic [/B]
I must mention value DC, I put in the picture as comment
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File Type: gif dc_offset.gif (3.5 KB, 179 views)
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Old 14th August 2009, 03:16 PM   #4
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You seem to have two different ground symbols! Are they connected?

I am sure your English is much better than my Portuguese, but I do not understand your words

If that is the complete schematic it cannot work.

You must have the power and feedback connected for the simulator to make sense!
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Old 14th August 2009, 04:37 PM   #5
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Quote:
Originally posted by cliff
You seem to have two different ground symbols! Are they connected?
Yes is connected
Quote:
Originally posted by cliff
I am sure your English is much better than my Portuguese, but I do not understand your words [/B]

Can you tell me which part did not understand, would happy if you tell me

Quote:
Originally posted by cliff
If that is the complete schematic it cannot work. [/B]
No, I put as in the book of Self, attached complete scheme..
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File Type: gif amp.gif (14.0 KB, 184 views)
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Old 15th August 2009, 11:48 AM   #6
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OK.

You probably have an error in your schematic.

Try this:

Disconnect the servo (remove the 100K resistor)

Make the input 0v, or ground it.

Measure the voltages on the input pair, E,B,C and list them here.
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Old 16th August 2009, 03:04 AM   #7
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Quote:
Originally posted by cliffforrest
OK.

You probably have an error in your schematic.

Try this:

Disconnect the servo (remove the 100K resistor)

Make the input 0v, or ground it.

Measure the voltages on the input pair, E,B,C and list them here.
The schematic is correct, I measured DC Offset in other schematics, directly in the differential pair.
- But, I put the wrong value, where is 32mA should read 32mV.
In this schematic can see the DC-offset, directly in the differential pair, the schematic is the book of the Self.
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File Type: gif dc_offset_selfbook.gif (13.0 KB, 92 views)
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Old 16th August 2009, 08:02 AM   #8
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Well, it is difficult to help if you give totally wrong information

Now what are you saying? 32mV on each base? + or- ? Or 32mV difference between bases?

Now I am not sure what your problem is?
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Old 16th August 2009, 12:14 PM   #9
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Rafael,
IF your circuit is correct, and IF you don't have any wiring errors, then non matched transistors in the diff pair will give you a mismatch. Just throwing in two transistors with the same part number is not good enough - we use monolithic duals in our equipment to avoid this problem: MAT02 for NPN, MAT03 for PNP. They have a guaranteed 50uV and 100uV max offset!

These are metal can and out of productoion, but AD have them in plastic cases now, same chip, lower price.

Regards, Allen (Vacuum State)
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